20 degrees is nothing to a chicken. No need to bother with insulation, because the birds already come with perfect insulation of their own, and don't need any help from us. And let them out of the coop. I've seen mine out and about in single degree temps, without a worry in the world. Check out the pic below. That's the front of my coop. The chickens have been in there, going on 5yrs, with no heat, no insulation, and no problem.